1. Field of the Invention
The present invention relates to a fuel control system for an internal combustion engine and, particularly, to a fuel control system for an internal combustion engine having a turbocharger, a bypass passage bypassing a compressor of the turbocharger, and an air bypass valve for opening and closing the bypass passage.
2. Description of the Related Art
Japanese Patent Laid-open No. H08-61073 discloses a turbocharging device for an internal combustion engine having a compressor for pressurizing intake air, a bypass passage for bypassing the compressor, and an air bypass valve for opening and closing the bypass passage. According to the device, the air bypass valve opens when the throttle valve is closed, and a portion of the intake air is recirculated to the upstream side of the compressor. The recirculation of the intake air reduces a rotating load of the compressor and increases the intake air flow rate to prevent the surging.
In the condition where the air bypass valve opens and the intake air passing through the compressor is recirculated to the upstream side of the compressor, the intake air flow rate detected by an intake air flow rate sensor on the upstream side of the region in which the bypass passage opens to the intake air passage may pulsate depending on a relationship between a timing at which the throttle valve closes and a timing at which the air bypass valve opens corresponding to the closing operation of the throttle valve. Further, since the intake air is recirculated, the flow rate of air actually supplied to the combustion chamber differs from the output of the intake air flow rate sensor. The amount of fuel supplied to the engine is usually calculated according to the output of the intake air flow rate sensor. However, if the fuel amount supply is calculated according to the output of the intake air flow rate sensor in the condition where the air bypass valve opens and the intake air passing through the compressor is recirculated to the upstream side of the compressor, the air-fuel ratio in the combustion chamber may deviate from a desired value, wherein the exhaust characteristics of the engine degrade.